Unemployment is rising and companies are going to the wall as the economic turmoil continues to inflict damage across the globe. But one organisation is thriving. Records recently filed at Companies House show Tony Blair Inc is going from strength to strength. They reveal that income channelled through a complex network of firms and partnerships controlled by Blair rose more than 40% last year to more than £12m. Of this, almost £10m was paid for "management services". The money was transferred via a network of firms and financial vehicles.

Accountancy experts are questioning the arcane nature of the network's finances, which makes it difficult to trace where its money is coming from, or where it is being spent.

Accounts for Windrush Ventures, an obscure company that operates under the trading name "the Office of Tony Blair", suggest 2011 has been a successful year for the former prime minister. Windrush saw its turnover rise to just over £12m, up from £8.5m in 2010. Pre-tax profits rose from £729,000 to £1.1m.

The accounts reveal that the company received "remuneration of £9,837,000 in connection with management services" from a limited liability partnership ultimately controlled by Blair. In the previous year Windrush Ventures Limited received £5.2m in remuneration for providing management services. Exactly what sort of management services are provided, and how the company derives its income, are impossible to determine as the accounts do not go into detail. Blair is legitimately taking advantage of laws allowing him to limit what his companies and partnerships must disclose. "It is baffling; these accounts make remarkably little sense," said accountancy expert Richard Murphy of Tax Research UK, a firm that scrutinises company finances. "This limited disclosure is not within the spirit of the law. "

Public documents have disclosed that a related financial vehicle, Windrush Ventures No3 LP, received almost $2.5m (£1.6m) from the Bill & Melinda Gates Foundation in 2008 to fund projects aimed at poverty relief in Sierra Leone. A Sainsbury family charity, the Gatsby Foundation, gave £992,000 in 2010 for charitable projects in Rwanda.

A spokesman for Blair explained the grants were in relation to his Africa Governance Initiative, now registered as a separate, independent charity. He declined to comment on how much, if any, of Windrush's income was derived from private business activities advising heads of state and global corporations. In the past year Blair has advised Nursultan Nazarbayev, Kazakhstan's president, who is reportedly keen to win the Nobel Peace Prize. It also emerged that in 2008 Blair wrote to the then Libyan leader, Colonel Gaddafi, discussing potential African infrastructure investments he should consider. In both cases, his spokesman denied Blair was personally profiting from the discussions.

Blair has been criticised for the way his private and philanthropic activities have appeared to merge. In his role as the Quartet's representative to the Middle East he helped persuade Israel to open up radio frequencies so a telecoms company, Wataniya Mobile, could operate in the West Bank. He also championed the development of a gas field off the coast of Gaza, operated by British Gas.

Both Wataniya Mobile and British Gas are major clients of JP Morgan, the US investment bank that pays Blair £2m a year for his role as a senior adviser. Blair said he had been unaware that both companies were clients of the bank and his spokesman stressed the deals were vital in bringing prosperity to the region.

Windrush Ventures Ltd spent almost £3m on staff, rent and other services but had total administration costs of almost £11m. "Just what is this company doing?" Murphy asked. "You would expect total costs to be around double the costs of employing staff. But in this case total administrative costs are £10.9m. That's a very high ratio indeed." He added: "We have no idea where this money is coming from or how it's being spent. The structure seems designed to impose a veil of secrecy over its accounts."

Blair has lucrative advisory roles for companies including luxury goods firm LVMH and Swiss insurer Zurich. He has undertaken work for the Kuwaiti royal family, an Abu Dhabi-based investment house and a South Korean oil firm. Much of his private business interests are thought to be channelled via another set of linked firms called Firerush Ventures. One of the firms has a licence from the Financial Services Authority allowing it to provide investment advice.

Blair's spokesman said: "Across all of his activities there are more than 120 people employed around the world. The Windrush accounts are prepared in accordance with relevant legal, accounting and regulatory guidance. Tony Blair continues to be a UK taxpayer on all of his income and all his companies are UK registered."